In recent years, rapid development of semiconductor technology has led to rapid progress of a reduction in size of semiconductor packages, the adoption of multipin, the adoption of fine pitch, the minimization of electronic components and the like. That is, the semiconductor field has entered the so-called “age of high density packaging.” This tendency has affected printed wiring boards, and the shifting of the wiring of the printed wiring boards from single side wiring to double side wiring, and, in addition, the adoption of a multilayer structure and a thickness reduction have been advanced (Iwata and Harazono, “Denshi Zairyo (Electronic Material),” 35 (10), 53 (1996)).
Pattern formation methods used in the formation of such wiring and circuits include: a method which comprises etching a metal layer, provided on a substrate in a layer construction of metal layer-insulating layer-metal layer, with an acidic solution, such as a ferric chloride solution, to form wirings, then subjecting the insulating layer to dry etching such as plasma etching or laser etching, or wet etching such as etching with hydrazine, to remove the insulating layer to form a desired shape for layer-to-layer continuity purposes (Japanese Patent Laid-Open No. 164084/1994), and connecting the wirings to each other, for example, through plating or electrically conductive paste; and a method (Proceedings of the 7th Symposium of Japan Institute of Electronics Packaging, issued in 1999) which comprises providing an insulating layer in a desired form using a photosensitive polyimide (Japanese Patent Laid-Open No. 168441/1992) or the like and then plating gaps to form wiring.
A tendency toward downsizing of electric products in recent years has led to a reduction in thickness of each layer constituting metal layer-polymeric insulator layer, and these layers each are in many cases used in a thickness of not more than 100 μm. When wiring has been formed of such thin layer, a warpage disadvantageously takes place in wiring due to a difference in coefficient of thermal expansion between the metal layer and the polymeric insulator layer.
When the thermal properties of the insulating layer and the conductor layer are known, the warpage σ of this substrate can be calculated according to the following equation (Miyaaki and Miki, NITTO TECHNICAL REPORT, 35 (3), 1 (1997)).
  σ  =                    3        ⁢                                  ⁢                  lE          1                ⁢                  E          2                            2        ⁢                  h          ⁡                      (                                          E                1                2                            +                              14                ⁢                                  E                  1                                ⁢                                  E                  2                  2                                            +                              E                2                2                                      )                                ⁢    ΔαΔ    ⁢                  ⁢    T  wherein
E1: modulus of the metal,
E2: modulus of the insulating layer,
Δα: difference in coefficient of thermal expansion between the metal and the insulating layer,
ΔT: temperature difference,
h: layer thickness, and
l: wiring length.
According to this equation, (1) a method wherein the modulus of the insulating layer is reduced, and (2) a method wherein the difference in coefficient of thermal expansion between the insulating layer and the metal wiring layer is reduced, are considered effective for reducing the warpage of wiring.
Regarding the wiring formation method, in the laminate used in the method for the formation of wiring through etching of a metal layer in the laminate having layer construction of first metal layer-insulating layer-second metal layer, in order to reduce the warpage of the laminate, the coefficient of thermal expansion of the metal layer should be made identical to that of the insulating layer. To meet this requirement, the use of a low-expansion polyimide as the insulating layer of the laminate has been proposed (U.S. Pat. No. 4,543,295 and Japanese Patent Laid-Open Nos. 18426/1980 and 25267/1977).
Since, however, the low-expansion polyimide is not generally thermoplastic, the adhesion to metal layers is so low that it is difficult to provide adhesive strength high enough to withstand practical use. A known method for overcoming this problem is to use a thermoplastic polyimide resin or epoxy resin having good adhesion to the metal layer as an adhesive insulating layer between the metal layer and the insulating layer (core layer) of the low-expansion polyimide (Japanese Patent Laid-Open No. 58428/1995).
Since the thermoplastic resin generally has a high coefficient of thermal expansion, the lamination onto a metal is causative of the warpage. To overcome this drawback, the thickness of the low-expansion core insulating layer having a coefficient of thermal expansion close to that of the metal is made larger than the thickness of the adhesive layer to avoid the appearance of warpage of the whole laminate on the surface of the laminate. The smaller the thickness of the adhesive insulating layer, the better the warpage preventive effect. When the thickness of the adhesive insulating layer is excessively small, however, the adhesion is deteriorated. At least when the total thickness of the adhesive layers respectively overlying and underlying the core layer is not more than the half of the thickness of the core layer, the warpage is less likely to occur. For this reason, for commercially available laminates fabricated for electronic circuit components, in many cases, the total thickness of adhesive insulating layers is not more than the half of the thickness of the core insulating layer. The formation of the adhesive insulating layer in a smallest possible thickness, which can ensure the adhesion, is regarded as ideal (Japanese Patent Laid-Open No. 245587/1989).
At the present time, rapid expansion of production of personal computers has led to increased production of hard disk drives incorporated in the personal computers. A component, in the hard disk drive, called a “suspension,” which supports a head for reading magnetism, is being shifted in its main products from one, wherein copper wiring is connected to a stainless steel plate spring, to one called a “wireless suspension” comprising copper wiring which has been connected directly to a stainless steel plate spring, from the viewpoint of coping with the size reduction.
The wireless suspension is mainly prepared using a laminate having a layer construction of first metal layer-adhesive insulating layer-core insulating layer-adhesive insulating layer-second metal layer. An example of the laminate is such that the first metal layer is formed of a copper alloy foil, the second metal layer is formed of a stainless steel foil, and the insulating layer is comprised of a core insulating layer and an adhesive insulating layer stacked on both sides of the core insulating layer. A wireless suspension using the laminate is scanned on a disk being rotated at a high speed and thus is a member to which fine vibration is applied. Therefore, the adhesive strength of wiring is very important. Accordingly, the wireless suspension using the laminate should satisfy strict specifications.
Hard disk drives are devices for recording information thereon. Therefore, a high level of data read/write reliability is required. To meet this requirement, the amount of refuse, such as dust, and outgas produced from the wireless suspension should be minimized.
A component called the “wireless suspension” is produced mainly by two methods, an additive method wherein wiring is formed by plating, and a subtractive method wherein wiring is formed by etching a copper foil. In the case of the subtractive method, only plasma etching by dry process is used for patterning of polyimide as the insulating layer.
A polyimide resin has been used as the adhesive for bonding between the insulating layer and the conductive inorganic material layer (metal layer) in the electronic circuit component, which satisfies the above strict specifications from the viewpoint of ensuring a high level of reliability of insulation. In order to impart adhesive properties to the polyimide resin, it is common practice to impart thermoplasticity. The introduction of a flexible structure, which can impart thermoplasticity, into a polyimide structure, however, in many cases enhances chemical resistance. Therefore, the polyimide resin, to which the adhesive properties have been imparted, is likely to have poor suitability for wet etching and is more difficult to be etched by wet process than the core insulating layer. For this reason, the insulating layers have been simultaneously etched by dry process using plasma or laser.
In the dry process, in general, sheet-by-sheet treatment is carried out. Therefore, the productivity is poor, and the apparatus is expensive. This disadvantageously leads to very high production cost. On the other hand, in the wet process, since a continuous object can be continuously etched, advantageously, the productivity is high and the apparatus cost is low. In wireless suspensions, however, the core insulating layer can be easily etched, while the adhesive insulating layer is difficult to be etched. Therefore, the adhesive insulating layer is left in a projected form (this phenomenon will be described later with reference to FIG. 9), and this poses a problem that a desired sharp etching shape cannot be provided and uneven etching occurs. The uneven etching is one of the causes of dusting during the use of wireless suspension. For this reason, in the laminate for wireless suspensions which should satisfy strict specifications, at the present time, the wet process cannot be put to practical use.
In the course of more detailed studies on the accuracy of the etching shape and the stability of the etched pattern in wet etching of the insulating layer, however, the present inventors have noticed that, in order to eliminate the above drawbacks, the conventional technique, that is, bringing the thickness of the adhesive insulating layer to the minimum layer thickness, which can keep the contemplated adhesion, is not an ideal one. The present inventors have found and aimed at the fact that, when the insulating layer in the laminate has been patterned by etching, irregularities (concaves and convexes) formed by the transfer of the shape of surface irregularities of the conductive inorganic material layer (metal layer) onto the adhesive insulating layer in the insulating layer affect the etching shape of the polyimide.
For example, a generally adopted process for producing electronic circuit components such as hard disk suspensions and flexible printed boards is as follows. An insulating layer is thermocompression bonded to and integrated with a conductive inorganic material (metal or the like) sheet to prepare a laminate. Alternatively, an insulating layer may be formed by coating on a conductive inorganic material (metal or the like) sheet to prepare a laminate. The laminate is then etched to produce the electronic circuit component. Various methods have been proposed for improving the interfacial adhesion of the laminate. Among them, a method, which is very effective and is generally used, is to utilize anchor effect. In this method, fine irregularities are formed on the surface of the conductive inorganic material layer. By virtue of the formation of the irregularities, at the time of compression bonding or coating, the insulating layer bites into the irregularities to develop the adhesion between the conductive inorganic material layer and the insulating layer. The transfer of irregularities of the conductive inorganic material layer onto the insulating layer can be confirmed by removing the conductive inorganic material layer in the laminate by etching or the like.
From the microscopic viewpoint, the formation of the irregularities in the insulating layer means that the thickness of the insulating layer varies from portion to portion. In the laminate used in the electronic circuit component, in general, the adhesive insulating layer formed on the surface of the insulating layer has in many cases a lower wet etching rate than the low-expansion core insulating layer. When the thickness of the adhesive insulating layer is identical to that of the core insulating layer, the time necessary for etching the adhesive insulating layer is in many cases longer than that necessary for etching the core insulating layer. In this case, when the thickness of the adhesive insulating layer is uneven, the shape of the end face of the adhesive insulating layer becomes complicated. As a result, the end face of the adhesive insulating layer is dropped, and this is causative of the occurrence of refuse. Further, after the adhesive insulating layer in its smaller thickness portion is removed, the core insulating layer only in its portion corresponding to the smaller thickness portion of the adhesive insulating layer is etched in an earlier stage than the other portion. As a result, the whole insulating layer cannot be evenly etched, and the etching shape is unstable.
When the thickness of the adhesive insulating layer is smaller than the average height of surface irregularities of the inorganic material layer in contact with the adhesive insulating layer, the irregularities of the inorganic material layer extend through some portions of the adhesive insulating layer and, consequently, disadvantageously, the adhesive insulating layer is partially absent. As in the above case, when the insulating layer in the laminate is etched, the etching shape of the insulating layer is uneven.
FIG. 1 is a flow diagram of the production of a laminate for an electronic circuit component, for example, by pressing, shown for comparison with the present invention, wherein a laminate is produced based on common knowledge of the conventional technique wherein the adhesive resin layer in the insulating layer is made as thin as possible. Specifically, in the production process shown in FIG. 1 (typical diagram), an insulating layer comprising a core insulating layer 1, an adhesive insulating layer 2 provided on one side of the core insulating layer 1 and an adhesive insulating layer 3 provided on the other side of the core insulating layer 1 is sandwiched between a first inorganic material layer 4 and a second inorganic material layer 5 (FIG. 1 (1)), the assembly is pressed to produce a comparative laminate (FIG. 1 (2)), and the second inorganic material layer 5 is removed by etching (FIG. 1 (3)). The laminate shown in FIG. 1 is an example of the case where the thickness of the adhesive insulating layer 3 in the insulating layer is equal to the average height of the surface irregularities of the second inorganic material layer 5.
FIG. 2 is a typical diagram showing the laminate, shown in FIG. 1 (3) with the second inorganic material layer 5 being removed by etching, masked by coating a part of the adhesive insulating layer 3 with a masking agent 6, wherein FIG. 2A is a cross-sectional view showing the layer construction of the laminate and FIG. 2B a top view of the laminate.
FIG. 3 is a schematic diagram illustrating an example of wet etching of the laminate shown in FIG. 2, wherein an etching process from the start of wet etching of the laminate, which has been partially masked with a masking agent 6, to the removal of the masking agent 6 to complete the etching is shown in the order of FIGS. 3A to 3D. FIG. 3A shows such a state that, except for the site masked with the masking agent 6, an etching liquid reaches the core insulating layer 1 and the core insulating layer 1 begins to be attacked by the etching liquid. In many cases, since the etching rate of the core insulating layer 1 is higher than that of the adhesive insulating layer 3, the core insulating layer 1 is rapidly attacked by the etching liquid. FIG. 3B shows such a state that, except for the site masked with the masking agent 6, the core insulating layer 1 has been substantially completely attacked by the etching liquid. FIG. 3C shows such a state that, except for the site masked with the masking agent 6, the adhesive insulating layer 2 has been substantially completely attacked by the etching liquid. FIG. 3D shows such a state that, after the completion of the wet etching, the masking agent 6 has been removed. Thus, in the wet etching, the difference in etching rate between the insulating unit layers constituting the insulating layers greatly affects the etching shape and poses a problem that the boundary of the etching shape is not smooth.
FIGS. 4A to 4D (FIG. 4) are top views corresponding to FIGS. 3A to 3D, respectively.
As shown in FIGS. 3 and 4, when the laminate produced based on common knowledge of the prior art technique, in which the adhesive resin layer in the insulating layer is made as thin as possible, is applied to wet etching, wavy unevenness of the etching shape of the insulating layer occurs.
The laminate for an electronic circuit component shown in FIGS. 1 to 4 is an example of the laminate in which the adhesive insulating layer in the insulating layer having a thickness which has hitherto been considered to be ideal for suppressing the occurrence of warpage and has been determined by taking into consideration the adhesion to the metal layer. As described above, the surface irregularities of the inorganic material such as the metal in the laminate contribute to an improvement in the adhesion of the inorganic material to the adhesive insulating layer. Since, however, the irregularities bite into the adhesive insulating layer to such an extent that is equal to the thickness of the adhesive insulating layer, when etching is carried out in this state, the etching shape after the removal of the masking agent is wavy (FIG. 4D). That is, a desired shape conforming to the mask cannot be provided, and the etching shape of the whole insulating layer is uneven. Thus, the accuracy is unreliable. The problem of the uneven complicated etching shape is likely to be led to dusting.
Accordingly, an object of the present invention is to provide a laminate comprising an insulating layer, which can realize an improvement in the shape of a patterned insulating layer, formed by patterning the insulating layer through wet etching, achieved by drawing attention to the state of irregularities transferred from the surface of the metal layer and, at the same time, is stable in the etching shape of the insulating layer and thus can suppress dusting, to provide an insulating film comprising the insulating layer, and to provide an electronic circuit component comprising a pattern of the insulating layer.